1. Field of the Invention
The present invention relates to catalysts which can be used in conversion reactions for hydrocarbons, in particular in process for reforming gasolines and producing aromatics.
2. Description of the Background
Catalytic reforming is a process which makes it possible to improve the octane number of the oil fractions and in particular of the heavy petroleum from distillation by conversion of n-paraffins and naphthenes into aromatic hydrocarbons.
The operation of catalytic reforming thus consists on the one hand of transforming C.sub.7 -C.sub.10 n-paraffins into aromatics and light paraffins and on the other hand C.sub.7 -C.sub.10 naphthenes into aromatics and light paraffins. These reactions are illustrated in particular by the conversion by dehydrogenation of cyclohexanes and the dehydroisomerization of alkylcyclopentanes to yield aromatics, methycyclohexane yielding for example toluene, and also by conversion by cyclization of n-paraffins into aromatics, n-heptane for example yielding toluene.
During catalytic reforming, cracking reactions also take place of heavy n-paraffins into light paraffins leading in particular to C.sub.1 -C.sub.4 products essentially of propane and isobutane: these reactions are detrimental to the yield of reformed product.
Finally, there is also the formation of coke through condensation of aromatic nuclei forming a solid product, rich in carbon which is deposited on the catalyst.
The reforming catalysts are very sensitive, apart from coke, to various poisons which can reduce their activity: in particular sulphur, nitrogen, metals and water.
By being deposited on the surface of the catalyst, the coke brings about a loss in activity with time which leads to higher operating temperatures, a lower yield of reformed products, and a higher gas yield.
Because of this and considering the regeneration of the catalyst, the catalytic reforming process can be put into operation in two different ways: in a semi-regenerating or cyclic manner and in a continuous manner. In the first case, the process is carried out with a fixed bed, in the second with a mobile bed.
In the semi-regenerating process, to compensate for the loss of activity of the catalyst, one raises the temperature progressively and then the installation is stopped in order to carry out the regeneration of the catalyst by eliminating the coke. In cyclic reforming which in fact is a variation of the semi-regenerating process, the installation comprises several reactors in series and each is closed down in turn, the coke deposits are eliminated from the catalyst out of action and the catalyst regenerated while the other reactors continue to operate.
In continuous reforming, the reactors put into operation are mobile-bed reactors operating at low pressure (less than 15 bars), which makes it possible to raise considerably the yields of reformed product and hydrogen by encouraging aromatization reactions instead of cracking, but on the other hand the formation of coke is greatly accelerated. The catalyst passes through the reactors then a regenerating action.
The processes for production of aromatics imply conversion reactions of the paraffinic and naphthenic hydrocarbons into aromatic compounds.
In these hydrocarbon conversion processes, there are generally used bi-functional catalysts containing, for example, platinum and a support of chlorinated alumina, which associate the acidic function of the chlorinated alumina necessary for the reactions of isomerization of cyclopentanic naphthenes and the cyclization of paraffins with the dehydrogenating function of the platinum necessary for the dehydrogenation reactions. Catalysts of this type, also including another metal such as rhenium, tin or lead, are described in US-A-3 700 588 and US-A-3 415 737.
As can be seen from the above, the catalytic reforming processes can be carried either by using a fixed bed or a mobile bed of catalyst.
In each case, the catalyst undergoes a regenerating treatment operating at high temperature and in the presence of steam, which consists among other things of burning off the coke deposited on the catalyst. Unfortunately, these treatment conditions favour degradation of the catalyst. It is thus important to try to raise the resistance of the catalyst under these conditions.
Generally the catalyst is presented in the form of extrusions or balls of a sufficient size to let the reagents and gaseous products pass relatively easily. Wear of the catalyst results, in particular through friction in processes with mobile beds, which provokes the formation of dusts and finer grains. These very fine grains perturb the gaseous flow and require raising the entry pressure of the reagents and even, in certain cases, to stop the unit. In mobile bed units, this progressive wear also has the consequence of perturbing the circulation of the catalyst and makes it necessary to top up the catalyst frequently.
A catalyst like a reforming catalyst must thus satisfy a great number of requirements, certain of which may appear contradictory. This catalyst must first of all provide the greatest activity possible allowing high yields to be obtained, but this activity must also be conjugated with the greatest selectivity possible, that is to say that cracking reactions leading to light products containing from 1 to 4 carbon atoms must be limited.
In addition, the catalyst must be highly stable vis-a-vis its deactivation through coke deposit; the catalyst must also have excellent resistance to degradation when it is submitted to the extreme conditions existing in the repeated regenerating operations it has to undergo.
In the case of the continuous reforming process operating for mobile bed reactors and as mentioned above, the catalysts are also submitted to intense and progressive wear through friction, which leads to a considerable diminution of their specific surface area and the formation of "smalls" which prejudice the functioning of the installation. The catalysts available at present, even if they can fulfill one or several of these conditions, do not satisfy the whole range of the requirements mentioned above.
Also, despite the many improvements already made to the bi-functional catalysts used, there is still a need for new catalysts offering improved performance, not only as far as the yield of conversion reactions is concerned, but also the lifespan of the catalyst.